The invention relates to a transmission system comprising a transmitter which includes a first coder for deriving a first coded signal from an input signal, a decoder for deriving a decoded signal from the first coded signal, determining means for determining a difference signal between the input signal and the decoded signal, at least a second coder for deriving at least a second coded signal from the difference signal and also transmitter means for transmitting the first and second coded signals by a transmit signal to a receiver, which receiver includes a first and a second decoder.
In addition, the invention relates to a transmitter, a receiver, a coder and a decoder to be used in such a transmission system, and a method of transmission, coding and decoding.
A transmission system as defined in the opening paragraph is known from the journal article "Low Bit Rate Transparent Audio Compression Using Adapted Wavelets by D. Sinha and A. H. Tewfik in IEEE Transactions on Signal Processing Vol. 41, No.12, December 1993.
Such transmission systems are used, for example, for transmitting speech or music signals by channels which have a limited transmission capacity.
A first example of such a channel is a radio channel between a mobile station and a fixed base station. The available transmission capacity of this channel is limited, because this channel is used by a great many users. A second example is a recording channel which utilizes a magnetic, optical or other recording medium such as, for example, a ROM. In that case the capacity is frequently limited too.
In the transmitter of the transmission system known from said journal article an input signal is converted into a coded signal by a first coder. The coded signal is converted into a decoded signal by a corresponding decoder in the transmitter. To improve the coding quality, the difference between the input signal and the decoded signal is determined, which difference signal is converted into the second coded signal by the second coder. The two coded signals are transmitted to the receiver where they are converted into a first decoded signal and a second decoded signal. By combining the two decoded signals in the combining means a reconstituted signal is obtained.
In prior-art transmission system the second coder is an adaptive wavelet transform coder. Such a coder is rather complicated. Besides, pre-echo signals occur due to the wavelet transform, which echoes detrimentally affect the perceptual quality of the transmission system.